editor: fix edition statement saving problem.

According to PO, the 'responsibilities' field from document 'edition
statement' should be optional in the editor form. This commit hides this
field (if it's empty) so, now, the user must choose to add this field
(and fill a value). Editing a document without 'responsibilities' is now
allowed by default.

* Closes #906

Co-Authored-by: Renaud Michotte <renaud.michotte@gmail.com>

rero_ils/modules/documents/jsonschemas/documents/document-v0.0.1.json

@@ -524,6 +524,9 @@
             "items": {
               "title": "Responsibility",
               "$ref": "#/definitions/language_script"
+            },
+            "form": {
+              "hide": true
             }
           }
         }